Orthogonal Frequency Division Multiplexing (OFDM) refers to a data transmission scheme wherein user-data is split into smaller data streams and transmitted using sub-carriers that each has a smaller bandwidth than the total available transmission bandwidth. The efficiency of OFDM results from the orthogonality of the sub-carriers. That is to say, the sub-carriers are selected such that they do not interfere with each other during transmission, thus resulting in an efficient transmission scheme.
Multiple-Input Multiple-Output (MIMO) refers to a wireless transmission and reception scheme wherein both transmitter(s) and receiver(s) employ multiple antennas for transmission and reception. A MIMO system takes advantage of the spatial diversity or spatial multiplexing options created by the presence of the multiple antennas to increase throughput.
A continuing challenge for OFDM-MIMO systems is system performance, i.e., capacity, reliability, etc. Towards this end, many techniques have been proposed for improving, for instance, channel capacity and/or reliability. An example of one such technique is referred to as “water-filling”, another example is power control. Water-filling and power control describe processes whereby a transmitter estimates channel conditions using feedback signals from a receiver in the system. Based on these estimates, the transmitter attempts to transmit user data in a way that optimizes channel performance in view of the channel conditions. As with similar techniques, water-filling and power control rely upon knowledge of the transmission channel, via feedback signals, to optimize channel performance. The signaling overhead associated with these feedback signals, however, is significant and often limits any potential increase in system performance. In addition, generating and transmitting feedback signals causes delays which also limit potential increases in system performance. These drawbacks to feedback signaling are particularly evident in systems with rapidly changing channel conditions, systems transmitting large amounts of data, and/or systems utilizing a large number of sub-carriers.
Accordingly, it is desirable to have a method and apparatus for efficiently estimating current channel conditions for use in improving overall system performance in OFDM-MIMO systems.